[PATCH 4.17 23/31] userfaultfd: remove uffd flags from vma->vm_flags if UFFD_EVENT_FORK fails

From: Greg Kroah-Hartman
Date: Sat Aug 04 2018 - 05:03:29 EST

4.17-stable review patch. If anyone has any objections, please let me know.


From: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>

commit 31e810aa1033a7db50a2746cd34a2432237f6420 upstream.

The fix in commit 0cbb4b4f4c44 ("userfaultfd: clear the
vma->vm_userfaultfd_ctx if UFFD_EVENT_FORK fails") cleared the
vma->vm_userfaultfd_ctx but kept userfaultfd flags in vma->vm_flags
that were copied from the parent process VMA.

As the result, there is an inconsistency between the values of
vma->vm_userfaultfd_ctx.ctx and vma->vm_flags which triggers BUG_ON
in userfaultfd_release().

Clearing the uffd flags from vma->vm_flags in case of UFFD_EVENT_FORK
failure resolves the issue.

Link: http://lkml.kernel.org/r/1532931975-25473-1-git-send-email-rppt@xxxxxxxxxxxxxxxxxx
Fixes: 0cbb4b4f4c44 ("userfaultfd: clear the vma->vm_userfaultfd_ctx if UFFD_EVENT_FORK fails")
Signed-off-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
Reported-by: syzbot+121be635a7a35ddb7dcb@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Eric Biggers <ebiggers3@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

fs/userfaultfd.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -631,8 +631,10 @@ static void userfaultfd_event_wait_compl
/* the various vma->vm_userfaultfd_ctx still points to it */
for (vma = mm->mmap; vma; vma = vma->vm_next)
- if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx)
+ if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
+ vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
+ }